This competing renewal continues to utilize human knee joints from donors across the adult age spectrum to establish phenotypic changes in articular cartilage and chondrocytes that are characteristic of normal joint aging and may predispose to the development of osteoarthritis (OA). The overall goal of this program is to begin with phenotype identification, proceed to elucidation of mechanisms and key cellular and molecular abnormalities in order to provide new directions for correcting aging-associated risk factors for OA and therapeutic interventions for established OA. Access to human tissues provides a unique opportunity to test the human and clinical relevance of novel basic and molecular mechanisms not only through correlative studies but also through an array of comprehensive in vitro studies with cells isolated from human tissues. The overall hypothesis proposes that cartilage aging is associated with loss and dysfunction that differentially affects cartilage cell subpopulations. Chondrocytes are compromised in function by aging-associated changes in signaling mechanisms and the inflammatory milieu of OA. The proposed program will continue the existing cores (A: Administration;B and C now combined into Core B: Tissue Acquisition, Morphology, and Cell Culture). The following 3 projects are proposed: Project 1, "Chondrocyte subpopulations in aging and osteoarthritis" (PI: M. Lotz) will characterize chondrocyte subpopulations and the role of Sox9-dependent activation of chondrocytes in normal and aging cartilage. Project 2, "Dysfunctional chondrocyte differentiation in aging cartilage" (PI: R. Terkeltaub) addresses the regulation of articular chondrocyte hypertrophy by altered Pi and PPi metabolism with focus on specific enzymes (NPP1, TNAP) and transporters (Ank, Pit-1). Project 4, "Mechanobiology of human articular cartilage degeneration: aging and osteoarthritis" (PI: R. Sah) will analyze biomechanical mechanisms of early and advanced cartilage degeneration and determine consequences for chondrocyte function and survival. Collectively, the results from the proposed studies will add important new information on chondrocyte biology, cartilage aging, and osteoarthritis pathophysiology.